1. Field of the Invention
The present invention relates to a device capable of separately controlling dB-linear gain in an analog circuit, and more particularly, to a gain control device not affecting a value of a negative feedback device connected with an operational amplifier in an analog circuit during gain control, and an amplifier using the gain control device.
2. Discussion of Related Art
In general, when many analog signal processing circuits, such as audio signal processing chips, wireless communication systems, and image sensor signal processing chips, perform amplification or attenuation on an input signal, gain of a gain control signal should be dB-linearly controlled to be amplified or attenuated.
The term “dB-linear” means that a log function of an output is linear when a magnitude of an input signal linearly increases. In other words, gain should be expressed by an exponential function with respect to a linear gain control signal in a gain control circuit.
A dB-linear gain control circuit may be implemented using an exponential function characteristic of a bipolar junction transistor (BJT) itself. Lately, most analog circuits are manufactured using a metal-oxide semiconductor field effect transistor (MOSFET) due to low-power circuit design and development of a complementary metal oxide semiconductor (CMOS) process. However, the MOSFET does not have the exponential function characteristic, and thus a circuit for implementing an exponential transfer function is required. Accordingly, various techniques for approximately performing dB-linear gain control have been disclosed.
FIG. 1 illustrates a general dB-linear gain controller, which is an example of a dB-linear programmable gain amplifier (PGA) using a MOSFET.
Referring to FIG. 1, a PGA 100 includes an operational amplifier (OPA) 120, negative feedback resistors 101 and 103, and switching means 111 and 113 controlling the resistances of the resistors 101 and 103 to vary, and is in a fully differential form.
An input/output transfer function of the PGA 100 may be expressed by Equation 1 below.
                              I          TP                =                                            1              +              x                                      1              -              x                                ·                      1                          R              REF                                ·          Vinp                                    <                  Equation          ⁢                                          ⁢          1                >            
Here, x denotes a gain control input signal variable, Vinp and Vinn denote input signals, and Voutp and Voutn denote output signals.
Referring to Equation 1 above, gain of input signal difference is
            1      +      x              1      -      x        .This value approximates 10x, and gain has an approximately exponential shape with respect to the gain control input signal variable x.
FIG. 2 shows graphs of a gain function
  y  =            1      +      x              1      -      x      of Equation 1 and an exponential function y=10x, illustrating a function of 20·log(y) with respect to x ranging from −0.7 to +0.7. Here, the graphs of y=10x and
  y  =            1      +      x              1      -      x      are denoted by reference numerals 201 and 203, respectively.
In FIG. 2, the graph 203 of
  y  =            1      +      x              1      -      x      is similar to the graph 201 of the exponential function y=10x with respect to x ranging from about −0.7 to +0.7. In other words, the two functions can approximate each other with respect to a specific range of x, and thus dB-linear gain control is enabled.
Lately, analog circuit design has been geared toward low power and low cost, and functional integration of analog signal processing units such as amplifiers, signal converters, and filters, which are dB-linear gain controllers mentioned above, is required. Thus, the necessity of a gain controller that can be functionally integrated with analog signal processing blocks such as a signal converter and filter, is increasing.
However, general dB-linear gain controllers need to adjust the value of a device negatively feeding back an output of an operational amplifier as an input.
For example, in the PGA 100, that is, the gain control device of FIG. 1, devices connected for negative feedback are resistors 101 and 103, and resistances of the resistors vary to adjust gain. When an analog signal processing unit such as a signal converter and filter is combined with the rear end of the PGA 100 constituted as mentioned above, a signal processing characteristic of the analog signal processing unit is changed by the resistors 101 and 103. Since a change of the values of the negative feedback devices 101 and 103 connected with the operational amplifier 120 leads to changes in an analog signal processing characteristic as well as gain control, it is difficult to combine an analog signal processing unit with the PGA 100.
To solve this problem, an amplifier capable of dB-linear gain control is added to the front end of an analog signal processing unit. However, this structure has an increased volume due to the additional circuit and thus is difficult to be miniaturized and highly integrated. Also, it is difficult to implement a low-power circuit due to increasing power consumption.